High-Performance Ligand-Protected Metal Nanocluster Catalysts for CO₂ Conversion through the Exposure of Undercoordinated Sites

Round 1

Reviewer 1 Report

In this short review, the author described complementary theoretical simulations which were performed to obtain atomic level understanding of the electrochemical reduction of CO₂ (CO₂RR) on ligand protected Au based nanoclusters, which is critical for rational design to achieve a more efficient conversion process of CO₂RR. There are some issues in this manuscript to be addressed.

1.In abstract, the author mentioned that 'A significant finding … is that the presence of all ligands in the nanoclusters leads to a dramatic decrease in catalytic activity …'. However, according to section 2.2.1, the catalytic activity could be enhanced by the defect in the materials introduced through the partial removal of ligands, which seems to be contrast with the statement in abstract. The author should carefully revise the abstract to make it clear and consistent with further descriptions.

2. In section 2.2.2, the author described the mechanism of incorporation of dopant into Au NC as an approach to facilitate CO₂RR, including monopalladium substitution and doubly Cd substitution. Related conclusions are suggested to be condensed and reiterated in the abstract.
3. The references should be updated. Recent advances on sustainability and hybrid catalysts (Small Methods 2021, 5, 2001250; Advanced Science 2020, 7, 1903730; VIEW 2021, 2:20200188) should be included.

There are some abbreviations need to be clear to address the common interest of this review, including 'SR' (appear in the name of materials), 'FE' (first mentioned in line 9 of page 5), and so on.

4. The author should check the manuscript carefully since there are a lot of small mistakes. For example, in line 3 of page 20, an extra 'the' should be deleted. And in line 7 of Summary and Outlook (on page 21), there is an extra comma following 'investigated'.
5. Some figures (including Figures. 2/6/7/8/9) are blurred, and the author should replace them with clear ones. Besides, if it is possible to combine related pictures in one figure, such as Figure. 6 and 7, to facilitate readers to compare the results?

Author Response

Please see the attachment in the box

Author Response File: Author Response.pdf

Reviewer 2 Report

In this review the authors have reported the catalytic role of ligand protected Au-based nanoclusters Au25(SR)18, Au24Pd(SR)18, Au23(SR)16 and Au21Cd2(SR)16 for electroreduction of CO2 to CO. Although observations are useful, there are several areas where improvements are needed before this manuscript can be considered for publication. See the comments below:

• Authors should provide electron microscopic images (HRTEM) to understand the morphological features of the nanoclusters.
• Future perspectives portion should be more elaborative with more future application potential for this electroreduction process.
• Authors should compare the activity of these Au-nanoclusters with related nanocatalysts for the electroreduciton of CO2.

Comments for author File: Comments.pdf

Author Response

Please see the attachment in the box

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper by Dominic Alfonso is a mini-review deals with the progress made by the author (and coworkers) to understand the experimentally observed reduction process on four gold clusters. A. The topic is clearly presented, and the paper is concise and enjoyable. The references are adequate. In my opinion this paper can be published in Catalysts, after fixing some typos.

For instance:

1. 12 row 9 from bottom about one order
2. 15 row 1 from top                 revealed
3. 17 row 4 from bottom for both NCs
4. 20 row 3 from top                 the the

Author Response

The 4 typos indicated by the referee have been corrected. The manuscript has been thoroughly proofread to correct additional typos that existed in the original manuscript.

Round 2

Reviewer 1 Report

The paper can be accepted.

Author Response

We thank the referee for recommending the paper for publication. The manuscript was thoroughly checked to address any grammatical errors. 

Reviewer 2 Report

In this manuscript author have reviewed the potential of ligand protected Au-based nanoclusters as heterogeneous catalysts for electroreduction of CO2 to CO. The role of undercoordinated S sites in designing reactive heterogeneous catalysts in terms of both activity and selectivity in the CO2 reduction processes is noteworthy. However, several areas need improvements. Thus, I recommend a revision to this manuscript considering the comments given below:

For most of the figures taken from previous literatures inside fonts are often too small. Image quality has to be improved.

For Figure 6 (I guess it is author’s own data), font size within the figure is too small.

For a comprehensive review on CO2 electroreduction over Au-nanoclusters, total number of references 68 is too small. Author should discuss related area of CO2 reduction and relevant reaction products more in the background. See and include these works:  Chem. Eur. J. 2018, 24, 7278-7297; Green Chem. 2020, 22, 4002-4033.

Author should provide representative HRTEM images of Au-nanoclusters with and without thiolate ligands for better understanding on the nanostructures responsible for enhanced catalytic activity.

Author Response

Comment 1: For most of the figures taken from previous literatures inside fonts are often too small. Image quality has to be improved. For Figure 6 (I guess it is author’s own data), font size within the figure is too small.

Response: Versions of Figures 3 and 4 having better image quality are provided. The STEM images are now shown as a separate figure (Fig. 5 in the current version). The same is true for the experimental data for the Au₂₅(SR)₁₈ and its Pd-doped counterpart (Fig. 6 in the current version). We find this to be necessary as combining them in a single figure would compromise the quality. Figures 8 and 9 (formerly 7 and 8) were also replaced by versions with enhanced quality.

The fonts in Figure 7 (formerly Fig. 6) were increased to enhance visibility. 

 Comment 2: For a comprehensive review on CO2 electroreduction over Au-nanoclusters, total number of references 68 is too small. Author should discuss related area of CO2 reduction and relevant reaction products more in the background. See and include these works:  Chem. Eur. J. 2018, 24, 7278-7297; Green Chem. 2020, 22, 4002-4033.

Response: The paper is  a mini-review of our group's previously reported Au based NC that specifically generates CO as the sole high-value product from CO₂. For a mini-review of previous efforts covering the performance of a subset of materials in terms of generating one specific product (including performance comparison with related Au based NC and NP systems that carry out the same limited albeit industrially relevant chemistry), the number of references provided may be arguably sufficient enough. We also note that the additional references that are recommended to be included cover materials that do not necessarily fall within the specific category of CO2 to CO reduction process. Materials that produce other hydrocarbons and alcohols are not intentionally considered in order to constraint the short review within what we feel is a reasonable length.

Comment 3: Author should provide representative HRTEM images of Au-nanoclusters with and without thiolate ligands for better understanding on the nanostructures responsible for enhanced catalytic activity.

Response: As indicated in our previous reply to this point, there was no attempt in our previously published efforts to generate STEM images for Au₂₅(SR)₁₈^- , Au₂₃(SR)₁₆^- and Au₁₉Cd₂(SR)₁₆^-. This is the reason we are not able to include them in the short review as they are unavailable. We have one though for the Pd doped Au₂₅(SR)₁₈^- which was included in the previous version (Fig 5 in the current version).

Supplementary Comment: We thoroughly went through the manuscript to brush up segments that would require moderate English changes.